Explanation

Unplugging a PDB
The following prerequisites must be met:
The current user must have SYSDBA or SYSOPER administrative privilege, and the privilege must be either commonly granted or locally granted in the PDB. The user must exercise the privilege using AS SYSDBA or AS SYSOPER at connect time.
The PDB must have been opened at least once.
The PDB must be closed. In an Oracle Real Application Clusters (Oracle RAC) environment, the PDB must be closed on all instances.
Dropping a PDB
The DROP PLUGGABLE DATABASE statement drops a PDB. You can drop a PDB when you want to move the PDB from one CDB to another or when you no longer need the PDB. - THIS MAKES C CORRECT.
When you drop a PDB, the control file of the CDB is modified to eliminate all references to the dropped PDB.
Archived redo log files and backups associated with the PDB are not removed, but you can use Oracle Recovery Manager (RMAN) to remove them.
When dropping a PDB, you can either keep or delete the PDB's data files by using one of the following clauses:
KEEP DATAFILES, the default, retains the data files.
The PDB's temp file is removed even when KEEP DATAFILES is specified because the temp file is no longer needed.
INCLUDING DATAFILES removes the data files from disk.
If a PDB was created with the SNAPSHOT COPY clause, then you must specify INCLUDING DATAFILES when you drop the PDB.
The following prerequisites must be met:
The PDB must be in mounted mode, or it must be unplugged.